The Influence of microstructure and microtexture on fatigue crack initiation and growth in α+β titanium

The strain paths and thermal cycles utilized during thermomechanical processing of two-phase alloys have a pronounced influence on the resulting distribution of grain orientations present and their spatial distribution. For example, large regions of similarly oriented α grains, commonly referred to as microtextured regions or macrozones,may persist despite the imposition of large macroscopic strains. The detrimental effect of microtexture on dwell fatigue life of high temperature alloys is well established; however,considerably less attention has been given to the effects of microtexture on fatigue life during continuous cycling . In the present work, the effects of microstructure and microtexture on the low cycle fatigue (N _f≤ 10⁴ cycles) behavior of Ti-6A1-4V have been characterized using electron microscopy. Microstructural parameters such as the volume fraction and size of the α phase were assessedby quantitative metallography while the contiguity of the α phase and the size and shape of the microtextured regions were investigated with electron backscatter diffraction. Variations in microstructure and microtexture due to subtle differences in thermomechanical processing routes have been correlated with variations in fatigue life through the use of quantitative fractography techniques. Using these methods the spatial and crystallographic orientations of fracture facets at small crack lengths have been determined. The results indicate that grains with c-axes are oriented between approximately 25° and 55° from the stress axis are most likely to form cracks that propagate by facet formation on the basal plane. Crack advance by faceted growth occurs readily through grains with similar basal plane orientation and,as a result,the contiguity of equiaxed α grains with basal poles in the 25°-55° range is an important parameter governing low cycle fatigue life.